The facts are clear. The non-club flyers have every right to use the rc frequencies. Maybe they are using the county land illegally, and can be forced out. That won't prevent them from setting up later on private land nearby, and with permission. They'll know who instigated their removal, and any negotiation will then be futile.

In the short term, a freq. agreement is the best course, but personally, I wouldn't be comfortable with that.

Nope, the club members will have to bite the bullet and go 2.4. Cost of doing business, so to speak.